1. Field of the Invention
This invention relates generally to the field of disk drives, and more particularly, to their digital motor speed control circuits.
2. Brief Description of the Prior Art
It is common practice to use a brushless D.C. motor to drive a disk drive spindle. The speed of rotation of the spindle must be maintained at a constant r.p.m., which is typically 3600 r.p.m. The speed regulation apparatus conventionally includes a sensor coupled to the motor shaft which outputs a pulse periodically on rotation of the shaft. The frequency of output of this pulse is then compared to the frequency of a reference. Based on the comparison, the speed control circuit either increases the speed of rotation or decreases it.
A typical speed control system, according to the above, is shown in Dohanich, Jr. U.S. Pat. No. 3,950,682 (filed Dec. 19, 1974) assigned to International Business Machines Corporation, Armonk, N.Y.